builtin/config: track "fixed value" option via flags only

builtin/config: track "fixed value" option via flags only

We track the "fixed value" option via two separate bits: once via the
global variable `fixed_value`, and once via the CONFIG_FLAGS_FIXED_VALUE
bit in `flags`. This is confusing and may easily lead to issues when one
is not aware that this is tracked via two separate mechanisms.

Refactor the code to use the flag exclusively. We already pass it to all
the required callsites anyway, except for `collect_config()`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>